Contact Information

Office of Biological and Environmental Research
U.S. Department of Energy Office of Science

(SC GG 5.21.1)
Develop predictive model for contaminant transport that incorporates complex
biology, hydrology, and chemistry of the subsurface. Validate model through field

First Quarter Results

This report summarizes the most recent results of the research
project “Stability of U(VI) and Tc(VII) Reducing Microbial Communities
to Environmental Perturbation: Development and Testing of a Thermodynamic Network
Model” that is attempting to model (predict) the effect of exogenous
chemical amendments on the subsurface microbial community at the Oak Ridge
Field Research Center (FRC). The model explicitly couples the thermodynamics
of microbial growth and geochemical reactions to make quantitative system-specific
predictions of microbial community dynamics. Model predictions are being tested
with the results of small- to intermediate-scale field experiments.

1. Overall Objective and Hypothesis
The overall goal of this project is to develop and test a thermodynamic network
model for predicting the effects of substrate additions and environmental
perturbations on the composition and functional stability of subsurface microbial
communities. The overall scientific hypothesis is that a thermodynamic analysis
of the energy-yielding reactions performed by broadly defined groups of microorganisms
can be used to make quantitative and testable predictions of the change in
microbial community composition and system geochemistry (including contaminant
chemistry) that occur when a substrate is added to the subsurface or when
environmental conditions change.

2. Types of Investigations Performed
Our research consists of three main activities:

Model Development: A list of the major microbial groups present
in samples of groundwater and sediment from the FRC has been compiled.
Computer programs have been developed to compute the overall growth reaction
and governing thermodynamic quantities for each group. Necessary model
input parameters (electron transfer efficiencies) are being estimated from
large numbers of published laboratory experiments. The output from this activity
is a thermodynamic data base containing the chemical stoichiometry and
standard state free energy change that defines the growth of each group (e.g.
denitrifiers, iron reducers, etc.). Collectively these calculations define
the growth reactions and energy flows in an intact microbial community.

Numerical Simulations: The thermodynamic data base is combined
with existing geochemical data and used to predict equilibrium reaction
paths that show the coupled changes in microbial community composition
and system geochemistry that occur when amendments are added to the subsurface.
Simulations are being performed to investigate the effect of ethanol
additions on uranium and technetium bioimmobilization for the major subsurface
environments at the FRC (Areas 1 and 3 with neutral pH and low nitrate groundwater;
and Area 2 with low pH and high nitrate groundwater). Inputs include measured
chemical quantities on sediment and groundwater; outputs include predicted
changes in groundwater and sediment chemistry and microbial community
composition. Model predictions have been consistent with field observations
and provide important insights into the role of specific microbial groups
(esp. denitrifiers) on overall system response. Model predictions are being
used to investigate alternate bioimmobilization strategies (choice of substrate,
sulfate additions, etc.) and to predict the long-term stability of bioreduced
uranium and technetium to changing environmental conditions.

Experimental Verification: Ultimately model predictions must
be verified by direct measurement and we have assembled a suite of lipid
and nucleic acid-based biomarkers for this purpose. We are developing
a “dictionary” that
will allow predicted growth of defined microbial groups to be detected and
quantified by one or more distinct biomarkers. Model predictions for long-term
experiments in small-scale microcosms and intermediate-scale physical models
and for short-term small-scale field experiments are being compared with biomarker
data collected on groundwater and sediment samples. Initial comparisons are
made on total biomass and groups with known functional genes. Particular emphasis
is placed on denitrifiers, sulfate reducers, and iron reducers as these make
up the largest portion (> 90 % in some cases) of the entire community
following substrate addition. We are actively collaborating with several
NABIR investigators to apply model simulations to other experimental
systems.

3. Main Results
The most recent results of our project are summarized here:

The predicted microbial community composition varies greatly
from one site to the next in response to electron donor additions.
Principal factors are the concentrations of available electron acceptors,
primarily oxygen, nitrate, sulfate, iron, and manganese.

Denitrification, fermentation, and sulfate-, iron-, and manganese-reduction
are the major microbial processes in all environments tested.

pH has a relatively small effect on microbial community response
to donor addition. Microbial bicarbonate production rapidly increases
the pH in initially low pH environments.

Low concentrations of uranium and technetium provide very little
energy to microorganisms and microbial uranium and technetium reduction
consume only trivial amounts of added electron donor.

Bioreduced uranium can be readily reoxidized by oxidized groundwater once
electron donor additions ceases; bioreduced technetium is more resistant
to reoxidation

4. Planned Activities:

The activities planned for the coming year include:

We are continuing to refine the numerical model to include
additional groups of microorganisms and additional microbial processes
to better align the model with observations from laboratory and field experiments.
In particular we wish to include groups that are involved in the oxidation
of sulfide, ferrous iron, and bioreduced uranium.

We will design a series of laboratory microcosm experiments to explicitly
test model predictions for a range of defined conditions. In these experiments,
sufficient data will be collected to monitor changes in geochemistry and
microbial community composition during both bioreduction and reoxidation.

We will collaborate with other NABIR investigators to test the model’s
utility for interpreting laboratory and field data and for transferring
results from one system to another.

A series of numerical simulations will be conducted to quantify the stability
of an intact microbial community to various environmental perturbations
including changing pH, nitrate concentration, and donor availability.

5. Project Significance: This project is clearly showing that the ability
to predict the effects of donor addition on change inmicrobial community composition
is essential for creating conditions that favor the long-term stability of
bioreduced uranium and technetium. Moreover, the ability of a microbial community
to maintain functional stability (i.e. maintain high rates of uranium and technetium
reduction) when subjected to various environmental perturbations is of critical
importance for the ultimate use of bioimmobilization at DOE legacy waste sites.
The longer-term significance of this project will be to provide a comprehensive
theoretical framework for designing and interpreting complex field experiments
and to aid in “bridging-the-gap” between basic research and field
applications.

Highlights

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